Micro ball grid array (.mu.BGA) refers to a surface mounting packaging technique whereby the solder ball array pattern is typically less than about 0.75 mm pitch. This is contrasted to conventional ball grid array technology whereby the pitch is at least about 1.0 mm. Pitch refers to the distance measured from the center of an adjoining solder ball to the center of the next adjacent solder ball.
In second level packaging of an electronic array module or chip carrier to a printed circuit board or card, the solder interconnection is rigid and cannot flex under thermal mismatch conditions. As ball to ball spacing shrinks to 0.75 mm pitch and less, the diameter of the ball must be reduced to achieve such spacing. This results in a lower stand off between the module and printed circuit board or card, resulting in less resilience to stress during expected thermal cycles due to the thermal mismatch between the module and the printed circuit board. The reliability of such a second level package interconnect is therefore jeopardized due to the loss of resilience to stress. Accordingly, .mu.BGA has seen limited industrial applications because of reliability concerns. Reliability can be addressed by employing epoxy compositions to adhere the .mu.BGA package to integrated circuit boards or cards in order to minimize the effects of thermal mismatch between the device and the circuit board or card. However, this is not especially satisfactory since the epoxy compositions cannot be reworked in that they are thermosetting polymer materials.